There have heretofore been known opaque laminated glass, which is produced by bonding at least two transparent glass sheets with an interlayer having such an opacifier as calcium carbonate or silica dispersed in a thermoplastic resin. Such opaque laminated glass can transmit light, but cannot permit the persons or objects existing behind them to be visually identified. Therefore, the opaque laminated glass has been used in the locations where privacy protection is required to be secured, such as lighting windows, bathroom doors and panels or wainscots for balconies (for example, refer to Japanese Patent Publication (examined) No. 2-56295).
However, such conventional opaque laminated glass using an opacifier causes the problem that uneven coloring is likely to occur due to agglomeration or poor dispersion of particles of the opacifier. In addition, there is also created the problem that the particles of the opacifier, which are present at an interface between each glass sheet and the interlayer, impair adhesion at the interface so that fine air bubbles are likely to be formed.
In order to solve such problems, there has been proposed an interlayer for opaque laminated glass consisting of (a) a polyvinyl acetal resin produced by acetalization of a polyvinyl alcohol having a degree of saponification of not less than 96 mol % with an aldehyde having 1 to 10 carbon atoms, (b) a polyvinyl acetal resin produced by acetalization of a polyvinyl alcohol having a degree of saponification of less than 96 mol % with an aldehyde having 1 to 10 carbon atoms and (c) a plasticizer (Japanese Patent Laid-open No. 6-263489).
However, opaque laminated glass described in Japanese Patent Laid-Open No.6-263489 does not yet have a lowered visible-light transmittance suited for privacy protection, and also undergoes decreases in its transmittances for other rays or light, or impairs the proper, fundamental characteristics required of laminated glass, when an attempt is made to get its visible light transmittance decreased to a lowered level suited for privacy protection, thus being not necessarily satisfactory.
Also, the laminated glass, as produced by bonding, for example, an interlayer composed of a polyvinyl butyral rein plasticized with a plasticizer, etc. between at least two transparent glass sheets has been widely used as windowpanes in automobiles and buildings, etc. Furthermore, such laminated glass offers the advantage that utilization of interlayers colored with a variety of coloring agents can facilitate the resultant tinted laminated glass to control or regulate the inner quantity of light.
Laminated glass using this type of interlayers has the fundamental characteristics required of the laminated glass, such as good weatherability, satisfactory adhesion between the interlayer and glass sheet, reasonable resistance to penetration or piercement by objects when subjected to external impacts, and adequate resistance to shattering or shatter-proofness when broken by external impacts, although the laminated glass encounters the problem of inferior sound-insulating property.
Particularly, such laminated glass shows a reduced sound transmission loss owing to the coincidence effect in the range of middle to high frequencies of about 2,000 to 5,000 Hz, and suffers from deterioration in sound-insulating property, wherein the term “coincidence effect” is understood to refer to the phenomenon in which when a sound wave enters a glass sheet, a transverse wave is propagated on the surface of the glass sheet due to the rigidity and inertia of the glass sheet and becomes resonant with the incident sound wave, thereby causing sound transmission. The coincidence effect is shifted to a higher frequency region according as the surface density of the laminated glass is smaller, that is, the thickness of the glass sheet diminishes.
In accordance with an increasingly growing demand for improved sound-insulating property, in recent years, there is required the laminated glass which can exhibit excellent sound-insulating property in addition to the fundamental characteristics as described above. Referring to an interlayer for improved sound-insulating laminated glass and such sound-insulating laminated glass, for example, Japanese Patent No. 2703471 discloses laminated glass using an interlayer for sound-insulating laminated glass of which interlayer comprises not less than at least two different plasticized polyvinyl acetal resin films being laminated.
In order to enhance the sound-insulating property of laminated glass, however, there has been strongly demanded an interlayer for laminated glass which can attain the enhanced sound-insulating property as compared with the conventional ones.
In addition, the conventional sound-insulating polyvinylacetal resin interlayers, with their increased contents of plasticizers, have caused the problem that blocking is likely to occur during storage or handling, and such sound-insulating polyvinyl acetal resin film has therefore been laminated on both sides with the polyvinyl acetal resin films with a reduced content of a plasticizer to form the three-layered film, thereby preventing the interlayer from causing blocking.
The laminated glass using the conventional sound-insulating interlayers, although it has successfully prevented deterioration in the sound-insulating property as caused by the above-described coincidence effect and has exhibited excellent sound-insulating property, has confronted the problem that its sound-insulating property gets impaired in cases where it is exposed to temperature rises caused by irradiation with sunlight, etc.